1. Field of the Invention
The present invention relates to high SBS threshold optical fibers.
2. Technical Background
Stimulated Brillouin Scattering (SBS) is a dominant nonlinear penalty in many optical transmission systems. In many systems, it is desirable to transmit a large amount of optical power through optical fibers, while maintaining a high signal to noise ratio (SNR). However, as the optical signal power launched into an optical fiber increases, the launch power may exceed a certain threshold power (SBS threshold) and part of the optical signal power will then be reflected due to SBS as a backward propagating signal. Thus, due to SBS, a large amount of the signal power can be lost due to reflection back toward the transmitter. In addition, the scattering process increases the noise level at the signal wavelength. The combination of decrease in signal power and increase in the noise both lower the SNR and lead to performance degradation.
At finite temperatures, thermal excitations in glasses occur similarly to those of phonons in crystals, and the interaction of these vibrational modes with low intensity signal light produces spontaneous Brillouin scattering. As the intensity of the signal increases above a threshold the vibrations become coherent. This coherence is manifest through a density (acoustic) wave in the material that acts through electrostrictive coupling to create a Bragg grating on the incident field. The acoustic wave changes the refractive index and enhances the reflected light amplitude through Bragg diffraction. Above the SBS threshold level of an optical fiber, much of the incident light is converted into backward propagating photons, reducing the SNR.